My Macbook Pro takes hours to boot up. Help!

macbook pro late 2011

intel i7

when pressing the on botton to boot

the mac would turn on load up the greay apple logo, then the loading bar would start to fill after filling about 20 percent it would disapear and the spining loding icon would contitue to spin

and i have not yet seen it load up yet

i have tried to press the command r

which i pick english it would connect to wifi then renstall something for about 1 hour i then tought it would be normal then but when i wold then restart and again it would not boot up after that

please help

thank you